1. Field of the Invention
This invention relates to an aircraft gas turbine engine afterburner flameholder and, more particularly, to a removable flameholder which can be removed from the rear of the engine through the tailpipe with the engine installed in the aircraft.
2. Description of Related Art
It is well known in the aircraft gas turbine engine art to provide thrust augmentation by burning additional fuel in an afterburner located downstream of the engine turbine. The afterburner generally includes means for dispersing a main flow of fuel together with a flameholder to which the flame may attach. The flameholder locally reduces the velocity of the gas stream and establishes a recirculation zone within the afterburner in order to sustain the flame which would otherwise blow out. The flameholder further provides an ignition zone and a low temperature rise zone which, in conjunction with fuel injected in parallel and additionally to a pilot fuel flow, provides staging to accomplish broad temperature and thrust modulation of the afterburner. One well known type of flameholder is made of sheetmetal and comprises two concentric annular flame rings arranged to diverge from each other in a downstream direction. Fuel may be introduced either uniformly upstream of the flameholder or in a plurality of locally concentrated zones so that the afterburning flame is maintained downstream of the trailing edges of the flame rings.
In order to provide for positive and uniform lightoff of the afterburner during all modes of augmented flight operation, pilot fuel may be introduced and ignited by means of a point source igniter. The pilot flame, in turn, operates to ignite the main fuel droplets. It is well known to introduce the pilot fuel to the afterburner by means of discrete jets situated around the flameholder. The pilot fuel jets are generally located intermediate the flame rings such that each pilot jet receives gas flow from the turbine exhaust through an inlet to the flameholder.
One afterburner flameholder of this type is described in U.S. Pat. No. 3,765,178 issued to Robert Hughes Hufnagle, et al. on Oct. 16, 1973, and assigned to the same assignee as the present invention. The afterburner flameholder of the Hufnagle, et al. patent includes an inner flame ring and an outer flame ring spaced radially outwardly from and concentric with the inner flame ring to form an annular flow passage for receipt of the turbine exhaust. Reference is made to the Hufnagle et al patent for backgound purposes.
Modern afterburners such as the one described in the Hufnagle, et al. patent are made from sheet metal, with elongated slots having closed semicircular ends that are conventionally referred to as racetrack shaped holes and have pilot and main spraybars inserted therethrough. The flameholder is captured by the spraybars and the spraybars must be removed before the flameholder can be removed from the engine. For many aircraft gas turbine engines, the top or upper spraybars can only be removed from the engine with the engine removed from the aircraft, such as is the case for the General Electric F404, F1O1, F11O, and J85 engines. The engine must be removed from the aircraft in order to remove the flameholder, which is a costly and time consuming procedure. A typical removal procedure for a conventional flameholder comprises:
1. Removing the engine from the aircraft; PA1 2. Removing the main spraybars (for example, 24 or more) from the afterburner casing, including breaking fluid fitting connections to the main afterburner fuel manifold; PA1 3. Removing pilot spraybars (for example, 6 or more) which includes breaking connections to the pilot fuel manifold; PA1 4. Removing the igniter; PA1 5. Unbolting the flameholder links from outside the engine; and PA1 6. Removing the flameholder from the engine.
Upon reinstallation of the flameholder, an engine test cell checkout run must also typically be made. The main and pilot fuel systems are also flow checked prior to this test cell run. Since the integrity of all the fuel connections cannot be checked on the aircraft due to the close proximity of the top half of the engine to the aircraft structure, this test cell run is often required by government procedures and is a good operating safety practice. Presently a typical flameholder replacement requires 39 manhours worth of related steps and procedures to replace a flameholder which itself is about a 1.5 manhour job.
There is great desire to have a removable flameholder that can be removed and replaced without having to remove the engine from the aircraft or the spraybars from the engine and run the required checkout procedures discussed above. Such a flameholder is highly desirable because it improves aircraft availability and reduces yearly engine maintenance hours and engine operating costs.